Problems with game center account in clash of clans

A long time ago, I made a game center account to which I attached my village. This account hasn't been used anymore, and I has significantly progressed with his game of Clash of Clans since then. I want to transfer that game to my new phone, but it won't let me. I made a new game center account and even a new icloud account so I could use the "link to another device" functionality in the game. When I try that, it says this village is attached to another game center account and that I have to login to that one to get the transfer code. I don't have a clue what the login/pw for that account was. How can I solve this?

Please help me.